A common problem encountered by modern armed forces during the confusion of battle, as evidenced during the Gulf War, is the phenomenon known as "friendly fire" in which forces accidentally fire on their own troops causing unnecessary casualties and fatalities. Instances of friendly fire often involve aircraft or helicopters accidentally firing on their own ground troops. In addition, such incidents often occur at night when visibility is poor. Conventional techniques employed to make ground troops and their equipment more easily identifiable and thus prevent such mishaps from occurring include placing visual identifying markers such as inverted letters or geometric designs on armored equipment such as tanks, armored personnel carriers, etc., and placing brightly colored tarps on such equipment. In addition, conventional two-way radios are employed to enable troops to communicate with one another and thus prevent such mishaps.
All of the aforementioned techniques suffer from drawbacks. The aforementioned visual markers and colored tarps are only visible during the day. Thus, they will not eliminate friendly fire incidents which might occur at night. Moreover, such markings and tarps are not readily visible by different concentrations of ground forces which are separated over relatively large distances. Furthermore, since markings and tarps are only visible by a pilot flying at relatively close distances to any equipment so marked, the possibility exists that a who pilot flies near an unidentified concentration of ground troops to see if they are so marked will be attacked if such troops turn out to be the enemy.
The drawback to using conventional two-way radios to identify troops is that if the unidentified troops are enemy troops and if they are monitoring the frequency over which such transmissions are made, they will be alerted to the presence of a potentially hostile force. In addition, since the initial transmission might have to be sent several times in order to contact the unidentified troops, and since a given period of time will in all likelihood elapse before a response is received, such lapses of time make it more likely that the unidentified troops might discover the presence of the troops sending the initial transmission and attack such troops while they are waiting for the response.
It is, therefore, an object of this invention to provide a two-way communications system which uses the transmission of optical pulses of a relatively narrow bandwidth to enable armed forces of one side to instantaneously determine whether unidentified forces are friendly or unfriendly without alerting such unidentified forces to the fact that they are being identified.